On a computer that is running Windows 7, the usable memory (RAM) may be less than the installed memory.

For example, a 32-bit version of Windows 7 may report that there is only 3.5 GB of usable system memory on a computer that has 4 GB of memory installed.

Or, a 64-bit version of Windows 7 may report that there is only 7.1 GB of usable system memory on a computer that has 8 GB of memory installed.

Note The amount of usable memory in the examples are not exact amounts. Usable memory is a calculated amount of the total physical memory minus "hardware reserved" memory.

To view the installed memory and the usable memory in Windows 7, follow these steps:

1. Click Start button, right-click Computer, and then click Properties.
2. View the Installed memory (RAM) value under System. For example, if it displays 4.00 GB (3.5 GB usable), this means that you have 3.5 GB of usable memory out of 4 GB of installed memory.

This is expected behavior on computers that are running Windows 7. The reduction in available system memory depends on the configuration of the following:

The devices that are installed in the computer and the memory that is reserved by those devices

The ability of the motherboard to handle memory

The System BIOS version and settings

The version of Windows 7 that is installed (For example, Windows 7 Starter Edition only supports 2 GB of installed memory.)

Other system settings

For example, if you have a video card that has 256 MB of on-board memory, that memory must be mapped within the first 4 GB of address space. If 4 GB of system memory is already installed, part of that address space must be reserved by the graphics memory mapping. Graphics memory mapping overwrites a part of the system memory. These conditions reduce the total amount of system memory that is available to the operating system.

If you're on Intel P55/X58 chipsets there also maybe other reasons for less physical memory installed being reported which differs from less physical memory being usable. You'd need to read and bookmark http://i4memory.com/ramdetect.